Tamoxifen has been approved for both the treatment of breast cancer and the prevention of breast cancer. I recently spoke with a patient with a family history of breast cancer who was going to meet with an oncologist and consider starting tamoxifen for prevention. I feel that it is important to make sure medications will work for you and so I shared information with this patient about how her genetics could impact her response to tamoxifen.
Tamoxifen itself has a very minor effect in treating or preventing breast cancer. For a patient to receive benefit from tamoxifen, the patient’s body must convert tamoxifen to a more active drug known as endoxifen. This conversion occurs primarily through an enzyme known as CYP2D6.
There are some individuals who make little to no CYP2D6 enzyme. Therefore, when treated with tamoxifen, these individuals convert less of the tamoxifen to the beneficial endoxifen. This naturally impacts one’s risk for breast cancer occurrence, recurrence, and treatment.
If you haven’t had genetic testing, how do you know your CYP2D6 status? You don’t. But you can look to your ancestry to see how concerned you should be. Individuals with the lowest risk of tamoxifen failure are those of Hawaiian/Oceanian descent with ~12% being considered intermediate or poor metabolizers. In Native Americans the chance is ~32%, Latinos ~36%, African Americans ~40%, and in both East Asians and European Caucasians ~45%. If these numbers make you want to know what type of CYP2D6 metabolizer you are, set up a free 15 minute consultation with me https://calendly.com/gene…/informational-call-for-patients
Whether you pursue genetic testing or not, if you take tamoxifen, you should also be aware that certain medications can also decrease CYP2D6 and therefore make tamoxifen less effective. The list includes common medications such as paroxetine, bupropion, and fluoxetine. If you want to maximize tamoxifen benefit, then avoid medications that inhibit the CYP2D6 enzyme.
